A perspective view of a prior art chuck is shown in FIG. 1. Chuck 10, suitable for use in a lathe, includes circular base portion 11, annular projection 12, fixed clamps 13 and 14, and movable clamp 15. Annular projection 12 axially projects from a front end surface of base portion 11 to support work 20. Fixed clamps 13 and 14 are positioned radially outwardly from annular projection 12 and are fixedly secured on the front end surface of base portion 11. Movable clamp 15 comprises generally L-shaped block 151, screw 152 and clamping portion 153. L-shaped block 151 is fixedly secured to an outer peripheral surface of base portion 11. Clamping portion 153, positioned between annular projection 12 and block 151, is fixedly secured on the front end surface of base portion 11. Screw 152 passes through a threaded bore in an upper portion of block 151 to engage and move clamping portion 153 radially inwardly. More specifically, rotation of screw 152 forces or bends an upper part of clamping portion 153 radially inwardly along dashed line 16. Dashed lines 16, 17 and 18 represent the three-dimensional X, Y and Z axes for chuck 10, respectively. Dashed line 18 passes through the radial center of base portion 11 and corresponds to the centroidal axis of annular projection 12. Radial center O.sub.M of base portion 11 is depicted in FIG. 2. Cut-out portion 121a is formed in annular projection 12 within the first quadrant defined by the X and Y axes. Pin member 19a is fixedly disposed within cut-out portion 121a. Pin member 19a is generally prismatic within generally rhombic bases so that a transverse section thereof is generally rhombic. However, only a portion of pin member 19a may be prismatic. Alternatively, pin member 19a may have rhombic transverse sections without being prismatic.
Work 20 includes discoid portion 21 and machining portion 22 fixedly secured on a front end surface of discoid portion 21. Cylindrical hole 211a is axially bored into a rear end surface of discoid portion 21 and has a diameter slightly larger than the longer diagonal distance of the rhombic base or rhombic transverse section of pin member 19a. Therefore, hole 211a may receive pin 19a. The longer diagonal distance is designated as D.sub.1 in FIG. 1 and is defined by the longer diagonal line of the rhombic base or rhombic transverse section of pin member 19a.
Fixed clamps 13 and 14 and movable clamp 15 preferably are equiangularly spaced. Furthermore, each inner surface of fixed clamps 13 and 14 and clamping portion 153 is preferably arc-shaped with a radius of curvature corresponding to the radius of curvature of the outer peripheral surface of discoid portion 21.
The sequence of mounting work 20 on chuck 10 is as follows. Work 20 is placed on an end surface of annular projection 12 so that pin member 19a enters hole 211a. Once work 20 is so positioned, fixed clamps 13 and 14 and clamping portion 153 surround work 20. The, screw 152 is turned to force clamping portion 153 radially inwardly by bending an upper portion thereof along axis 16. As a result, clamping portion 153 firmly clamps discoid portion 21 against fixed clamps 13 and 14. Referring to FIG. 2, the solid and dashed lines represent before clamping and clamped situations, respectively.
However, during the clamping step of discoid portion 21 wherein clamping portion 153 forces discoid portion 21 against fixed clamps 13 and 14, work 20 may undergo undesirable motion. More specifically, work 20 may rotate about the longitudinal axis or rhombic center point Op of pin member 19a in response to the bending force of clamping portion 153. Such undesirable angular displacement is designated by .alpha. in FIG. 2. In turn, such angular displacement of work 20 would shift the radial center of work 20 from O.sub.W O'.sub.W as illustrated in FIG. 2. As a result, the radial center of work 20 may not be aligned with radial center O.sub.M of base portion 11 and chuck 10. Accordingly, accurate of precise machining of portion 22 is sacrificed. Consequently, the above described prior art chuck would not be suitable for machining parts such as scrolls used in scroll type fluid displacement apparatus wherein dimensional precision is critical.